In CSP plants, solar energy is captured in a concentrated way by means of mirrors, such as a field of parabolic troughs, or a combination of a tower and a field of heliostats. The thus collected solar thermal energy is used to heat a fluid that serves to produce steam which, in turn, is used to produce electric power by means of a turbine and generator system.
It will be understood that sun radiation is not available continuously. Therefore, the thermal energy recovered from solar radiation during day time, is stored during day and used during night time so as to allow the resulting power to be available at all times. This system, which effectively uses a charging phase and a discharging phase, also serves to accommodate diurnal changes in solar radiation.
The most common method to store the captured sun radiation energy is to heat, directly or via a heat transfer fluid, a mass of molten salts, mostly a mixture of nitrates, during the day, while using these hot molten salts, either directly or indirectly, for producing steam, so as to generate electric power (generally in a conventional Rankine cycle).
The heat storage systems used to store molten salts usually comprise one or more paired tanks (named “hot” and “cold” storage tanks). During molten salts heating, the molten salts are transferred from the cold tank to the hot one. When the heat is recovered, molten salts flow from the hot tank to the cold tank.
As an alternative to the two tank storage system, thermocline storage systems can be used. A thermocline storage tank system is a single-tank system containing both the hot and cold molten salts. This type of system relies on thermal buoyancy to maintain thermal stratification and discrete hot and cold thermal regions inside the tank. Since the density of high temperature molten salts is lower than that of low temperature molten salts, the first volume of high temperature molten salts stratifies on the top of the low temperature molten salts, thus forming a natural interface region extending substantially horizontally. It will be understood that, depending on the relative volumes of the high and low temperature molten salts, this interface moves substantially vertically relative to the storage tank. This system represents an economical alternative to the two-tank storage system.
According to the different CSP plants schemes, the “cold” tank (or the low temperature volume in a thermocline tank) operates within a temperature range varying from 270° C. to 400° C., while the hot tank (or volume) temperature may reach a maximum value of 550° C.
A background reference on heat storage from solar fields is WO 2012/107811. Herein a solar energy storage system is described including three or more reservoirs. This multi-reservoir thermal storage system contains a molten salt and/or molten metal thermal storage fluid. Essentially, the thermal storage fluid is heated by steam, tapped of from steam that is heated by insolation and used in a Rankine cycle. The disclosure is limited to solar plants having a configuration of a field of heliostats reflecting solar radiation, to a tower having solar receivers. Also, the disclosure related to the reservoirs is limited to thermal storage fluids that are not themselves heated by solar radiation.
The present invention pertains to solar fields of the type using parabolic troughs to collect solar thermal energy, having molten salts (MS) as a primary thermal medium. In these applications MS acts both as medium for the solar collecting field and as a liquid phase sensible heat thermal storage material. Because no extra heat exchange is involved, this approach is sometimes called direct thermal energy storage. As a result of this direct thermal energy storage from the solar field, the molten salts reach temperatures up to 550° C.
The system of direct thermal energy storage from solar fields, has advantages in terms of the efficiency of the Rankine cycle, and leads to an increase of the energy stored with MS as compared to systems involving an additional heat transfer fluid.
However, the high temperatures of the molten salts as a result of direct heating, also bring about drawbacks. One is the cost of the storage infrastructure, as tanks are needed that are capable of withstanding temperatures up to 550° C. Also, whilst the avoidance of additional heat transfer is advantageous, the heat exchange from the directly heated molten salts is subject to improvement.